In Dense Wavelength Division Multiplexing (DWDM), multiple light beams, each of a different wavelength and representing a distinct channel for the transmission of data, are combined (multiplexed) to propagate as a beam along a single optical beam path, such as a beam path defined by an optical fiber. The amount of information that can be carried along the beam path, e.g., by the fiber, is thus greatly increased. At the receiving end of the beam path the channels are de-multiplexed and appropriately demodulated. Each channel employs a laser light source, typically a semiconductor laser, such as a distributed feedback (DFB) laser or a distributed back reflection (DBR) laser, that produces a beam at the wavelength of that channel. A modulator modulates the beam to carry the channel's data. The development of a practical wide band amplifier that can be inserted in the optical beam path, such as the erbium doped fiber amplifier, has made DWDM a reality and spurred much technical innovation in related devices, such as multiplexers, demultiplexers, modulators, etc.
One important concern with DWDM systems is achieving higher data rates, such as by increasing the number of channels. The wavelength stability of the laser sources limits number of channels. The wavelength of a laser light source typically drifts over time, and the channels cannot be so closely spaced such that the wavelength of one channel laser source drifts too close to the wavelength at which another channel light source is operating. Information will be lost. Accordingly, the better the stabilization of the wavelength of the laser sources, the more densely the channels can be packed within a particular wavelength range.
For example, the wavelength of a DFB laser is known to be affected by several factors, such as laser source current, laser temperature, and aging of the laser. In most practical applications, the wavelength of the laser is stabilized by regulating the temperature of the laser, because changing the current affects the overall system power budget and provides a more limited range of wavelengths over which the laser can be tuned. DFB lasers are typically temperature stabilized using a thermal control loop consisting of a thermistor to sense the device temperature, an electronic feedback loop, and a thermoelectric cooler (TEC) that responsive to feedback adjusts the temperature of the laser. Thermal regulation is employed because it also protects the DFB laser from overheating, and helps to stabilize power output of the laser. However, laser drift is still a concern and limits the density of channels. Improvement is required to more densely pack channels, and hence obtain higher data rates, in DWDM systems.
Another important concern in implementing a DWDM system is wavelength management and optimization. System designers face difficult problems when optimizing a DWDM link. They need to minimize losses, yet maintain adequate channel isolation and consider other parameters relating to wavelength. Several components within a DWDM system, such as optical amplifiers (e.g. an erbium doped fiber amplifier), multiplexers, demultiplexers, optical isolators, add/drop multiplexers and couplers, are sensitive to wavelength. Fiber dispersion is also a consideration. Control, e.g., tuning, of the wavelength of individual channels within available channel bandwidths is not typically fully realized as an optimization tool.
Yet another concern in operating such systems involves monitoring the laser radiation used for some, or all, of the channels. As noted above, the wavelength is known to vary with the electrical current supplied to the laser, the temperature of the laser, and with the aging of the laser. Monitoring of the wavelengths can be useful in maximizing performance of the overall information transmission system.
The problems of wavelength regulation, control, and monitoring have not been satisfactorily resolved. Better wavelength monitoring, regulation and control will allow higher performance laser information systems that are more readily designed, maintained and modified, and denser packing of channels, and hence higher data rates. Fewer types of lasers could achieve a given number of communication channels. Existing methods and apparatus are not entirely adequate.
Accordingly, it is an object of the invention to address one or more of the aforementioned disadvantages and drawbacks of the prior art.
Other objects of the invention will in part be apparent and in part appear hereinafter.